.TH std::variant_alternative,std::variant_alternative_t 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::variant_alternative,std::variant_alternative_t \- std::variant_alternative,std::variant_alternative_t

.SH Synopsis
   Defined in header <variant>
   template <std::size_t I, class T>                          \fB(1)\fP \fI(since C++17)\fP
   struct variant_alternative; /* undefined */
   template <std::size_t I, class... Types>                   \fB(2)\fP \fI(since C++17)\fP
   struct variant_alternative<I, variant<Types...>>;
   template <std::size_t I, class T> class                    \fB(3)\fP \fI(since C++17)\fP
   variant_alternative<I, const T>;
   template <std::size_t I, class T>

   class variant_alternative<I, volatile T>;                  \fB(3)\fP \fI(since C++17)\fP
   template <std::size_t I, class T>                              (deprecated in C++20)

   class variant_alternative<I, const volatile T>;

   Provides compile-time indexed access to the types of the alternatives of the
   possibly cv-qualified variant, combining cv-qualifications of the variant (if any)
   with the cv-qualifications of the alternative.

   Formally,

   2) meets the TransformationTrait requirements with a member typedef type equal to
   the type of the alternative with index I
   3) meets the TransformationTrait requirements with a member typedef type that names,
   respectively, std::add_const_t<std::variant_alternative_t<I,T>>,
   std::add_volatile_t<std::variant_alternative_t<I,T>>, and
   std::add_cv_t<std::variant_alternative_t<I,T>>

.SH Member types

   Member type Definition
   type        the type of Ith alternative of the variant, where I must be in [0,
               sizeof...(Types)), otherwise the program is ill-formed.

   Helper template alias

   template <size_t I, class T>
   using variant_alternative_t = typename variant_alternative<I,          \fI(since C++17)\fP
   T>::type;

.SH Example


// Run this code

 #include <variant>
 #include <iostream>

 using my_variant = std::variant<int, float>;
 static_assert(std::is_same_v
     <int,   std::variant_alternative_t<0, my_variant>>);
 static_assert(std::is_same_v
     <float, std::variant_alternative_t<1, my_variant>>);
 // cv-qualification on the variant type propagates to the extracted alternative type.
 static_assert(std::is_same_v
     <const int, std::variant_alternative_t<0, const my_variant>>);

 int main()
 {
     std::cout << "All static assertions passed.\\n";
 }

.SH Output:

 All static assertions passed.

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to              Behavior as published              Correct behavior
   LWG 2974 C++17      out-of-bounds index resulted in undefined       made ill-formed
                       behavior

.SH See also

   variant_size                   obtains the size of the variant's list of
   variant_size_v                 alternatives at compile time
   \fI(C++17)\fP                        \fI(class template)\fP (variable template)
   std::tuple_element<std::tuple> obtains the type of the specified element
   \fI(C++11)\fP                        \fI(class template specialization)\fP
